Various means have been used by the printing industry to secure printing blankets to the outer surfaces of printing cylinders. A typical means is that in which the cylinder has a gap formed in its outer surface which extends inwardly to form a pocket or recess in which one or more reel rods are rotatably mounted. These reel rods generally have an axial groove in their surfaces for receiving the ends of the printing blanket. The blanket ends are reinforced with a strip of metal known as blanket bars, which stiffen the blanket end, making it easier to insert into the reel, rod grooves. After insertion of the ends into the grooves of the reel rods, the reel rods are rotated in a direction away from the gap so as to pull the blanket into the pocket and tighten it over the surface of the printing cylinder. See for example, U.S. Pat. Nos. 4,426,931 showing a dual reel rod system and 4,510,868 showing a single reel rod system.
These lock-up means work adequately, however, they do suffer several drawbacks, one of which is termed "blanket pullout." Blanket pullout refers to the tendency of the blanket ends to pull out of the groove of the reel rods, causing the blanket to separate from the printing cylinder. The result of blanket pullout is a damaged blanket which often must be replaced; substantial down time of the press while the blanket is inspected and reattached or replaced; and the loss of the printed material affected by the loose blanket.
This phenomenon has become more problematical in the "narrow gap" technology which is now being developed. Narrow gap refers to the cencept of reducing the size of the gap in the cylinder. Narrow gap cylinders generally have a gap width of less than 0.210 inches and ideally, a width of approximately 0.180 inches. Gap width on a conventional cylinder is generally about 0.250 inches. Narrowing the gap reduces the incidence of "bounce" during printing. Bounce is a vibration caused by the impact of the edges of the gap against the adjoining printing roll. Bounce affects printing quality, causing streaks and runs, and machine life, causing an increase in the rate of wear on the blankets and cylinders. Using a narrow gap also increases the number of available print lines for a given cylinder size. Because of these benefits, narrow gap technology has become increasingly more popular.
As the gaps have narrowed, the thickness of the printing bars have correspondingly decreased. These thinner bars are more difficult to hold in the reel rods and exhibit a significantly greater occurance of blanket pullout than blankets with conventional bars. Various devices have been designed or suggested to cure the problem of blanket pullout in narrow gap cylinders.
One such suggestion has been to reduce the width of the groove in the reel rod to form a tighter hold on the blanket end. This suggestion has not yet been proven or accepted in practice and would require replacement of the entire reel rod and would make insertion of the blanket end into groove more difficult.
Another attempt is that shown in U.S. Pat. No. 4,261,262 where one or more support rods are placed between the two reel rods to lock the blanket ends in the grooves of the reel rods and prevent their pulling out. This requires a redesigning or new purchase of the press cylinder to accommodate the additional support rods. Further, the support rods interfere with the easy insertion and removal of the blankets from the reel rods.
The present invention overcomes these difficulties and provides an easy, effective and inexpensive means for preventing blanket pullout, especially in narrow gap cylinders.